Likely a combination of 4 things:
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They have third party firmware in their blobs that they are under NDA regarding the source code.
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They believe in the source code is a large part of their success and don't want to reveal it.
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They believe giving out the source code will allow many inferior variants of the software, impacting their brand.
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Control; the more source code they have in mesa the more of their code can be rejected by mesa. Keeping their stuff as blobs allows them to put in whatever hacks they want.